Environmental contaminants contained in emissions from coal-fired and oil-fired power plants are a major environmental concern. Particulate matter (e.g., fly ash), nitrates, sulfates, and mercury emissions are restricted because these emissions can yield, for example, acid rain and serious neurotoxic effects. The removal of particulate matter has been addressed through the installation of baghouses, electrostatic precipitators, cyclone separators, or cyclone separators with baghouse filters in the flue gas ducts. The removal of nitrates and sulfates has been addressed through the addition of lime (calcium oxides and/or hydroxides) to the flue gas and the collection of the lime reaction product (e.g., CaSO4) with the particulate matter. The removal of mercury can be addressed by absorption with a mercury absorbent material. Unfortunately, the mercury absorbent materials and lime are often chemically incompatible and/or the mercury absorbent material is fiscally incompatible with the collection and disposal of the particulate material which is often sold into the concrete industry.
The most common method for reduction of mercury emissions from coal-fired and oil-fired power plants is the injection of powdered, activated carbon into the flue gas stream. The activated carbon is a high surface area material that provides for the adsorption of the mercury and the agglomeration of the particle bound mercury. The disadvantage of adding activated carbon into the flue gas stream is the retention of the activated carbon in the fly ash waste stream. Fly ash from coal-fired power plants is often added to concrete, where the presence of the activated carbon adversely affects performance, thereby making the inclusion of the carbon fiscally incompatible with the flue gas scrubbing process.
Another method for reducing Hg emissions is through the addition of chemical species that react with mercury to chem-adsorb the elemental and oxidized mercury. One class of materials capable of chemically reacting with Hg is metal sulfides. U.S. Pat. Nos. 6,719,828 and 7,578,869 teach the preparation of supported metal sulfides. A major disadvantage of these supported metal sulfides is that these materials are known to react with lime, for example, yielding copper metal and calcium sulfide materials. See e.g., Habashi et al., Metallurgical Transactions, 1973, 4, 1865. These reaction products destroy the absorptive capacity of mercury from the flue gas. Therefore, the use of supported metal sulfides has been physically separated from the use of lime in the flue gas scrubbing process.
There is still an ongoing need to provide improved pollution control sorbents and methods of their manufacture, particularly sorbents that are stable both in acidic flue gases and in the presence of acid reducing species.